This invention relates to an improved, highly reliable heat pack which is capable of passing from the super-cooled aqueous salt solution phase to a crystalline substantially solid phase with the generation of heat, and to the method of the subsequent recharging or regeneration of the super-cooled solution by raising the temperature of the substantially solid contents of a spent heat pack by contacting the contents with a source of microwave energy to thereby melt the solid to the liquid phase.
Heat packs which utilize a super-coolable aqueous salt solution that can be activated to liberate heat have become increasingly popular in recent years. Such heat packs as those shown in U.S. Pat. Nos. 4,077,390 and 4,572,158 and others, have significant advantages over former means of providing localized heat such as a hot water bottle or a heated electric element because of their portability and performance on short notice without the need for a supply of hot water or electricity to generate the heat. A super-cooled heat pack in its liquid state can be transported readily and is available on short notice to provide localized heat, as for the relief of pain and soreness in parts of the human body, as well as to warm parts of the body that have been subjected to extreme cold. Heat packs of this type are particularly useful in hospitals, clinics and in the home for the convenient generation of localized heat.
Up to now, it has always been necessary to place the spent substantially solid heat pack in a source of hot water and hold it there for a sufficient length of time and at a sufficiently high temperature of the water to melt the solid and return the contents of the heat pack to the liquid state. When that function has been accomplished the heat pack is ready for and capable of being reused, and the process can be repeated any number of times. Many users of such heat packs find the process of regeneration by immersion in hot water as described above to be inconvenient because of the time, appliances and care required to regenerate the pack without burning it. The result is that too often the discharged heat pack is set aside in the crystalline or solid state and is therefore useless for the quick application of localized heat since the user must first follow a lengthy process to regenerate or melt the contents in order to make it usable.
The foregoing disadvantage of heat packs known and used up until the present invention is a particular disadvantage in a hospital or clinic, particularly with the larger packs, where it becomes necessary to resort to the expense and use of the valuable time of hospital personnel to regenerate the spent heat packs to make them ready for reuse. As a result of this disadvantage, sales to this market have been significantly curtailed.
Flexible plastic heat packs of the type referred to above are shown in such U.S. patents as U.S. Pat. Nos. 4,379,448 and 4,460,546. The heat packs shown in these patents are unsatisfactory for microwave regeneration because such packs are known to contain internal welds which cause confined spaces within the pack resulting in hot spotting and melting or weakening of the flexible plastic pouch which thereby renders the heat pack unusable. In fact, such heat packs actually have carried a warning notice that the contents should not be microwaved to recharge or regenerate them.